/Engine/Source/Runtime/Core/Public/Math/CurveEdInterface.h
Set the incoming and outgoing tangent for the given subcurve and key. SubIndex must be within range ie >=0 and < NumSubCurves. KeyIndex must be within range ie >=0 and < NumKeys.
virtual void SetTangents ( int32 SubIndex, int32 KeyIndex, float ArriveTangent, float LeaveTangent )